Knocking of internal combustion engines may occur under certain operating conditions. Knocking, as well known, is considered to result from pressure waves or pulses of the fuel-air mixtures which can be heard as oscillations of the engine. Knocking causes high thermal loading of the walls of the cylinder and of the piston, which may also cause removal of material therefrom, and damage to the engine. It is, therefore, desirable that knocking be avoided. Continued operation of the engine under knocking conditions may lead to destruction thereof. The working range of the engine should be utilized as efficiently as possible, however, and it is thus desirable to operate the engine as close to the knocking limit as possible without, however, permitting knocking actually to occur. It is, therefore, desirable to be able to determine knocking or incipient knocking as quickly and as early as possible.
Various types of systems have been proposed to determine incipient occurrence of knocking in the combustion chamber of an internal combustion (IC) engine. It has been proposed to include an ion current measuring path within the combustion chamber of the IC engine in order to determine, and sense, pressure variations occurring in the combustion chamber--see, for example, German Patent Disclosure Document DE-OS No. 28 02 202. This system utilizes an evaluation circuit in which the ion current is transformed into a control signal which can be processed in a control system.
It has also been proposed to generate a control signal which is applied to a demodulator circuit. Such a system is described, for example, in German Patent Disclosure Document DE-OS No. 29 18 420, in which the knocking sensor generates a signal which includes background noise signals as well as signals representative of knocking oscillations. This signal, after demodulation in the demodulator circuit, is applied to two branches. One is the background noise branch, and one is the knock signal branch. The signals are applied to a comparator. The background noise signal is generated by a low-pass filter. The background noise signal, after filtering, is compared with the knocking signal in the comparator, which then provides a "yes/no" output, representative of whether knocking occurred, or not. A transducer is used which senses oxcillations of the IC engine. The evaluation circuit for the type of transducer there disclosed cannot be used directly to evaluate signals derived by sensing ion current variations.